The last time a blue moon happened was on New Year's Eve 2009. The next blue moon will occur July 31, 2015, according to moongiant.com.

The phrase "blue moon" has been around for more than 400 years, wrote
Philip Hiscock, a folklore expert at Memorial University of
Newfoundland, in an exhaustive piece on the origin of the phrase "blue
moon" over the years.

But the current meaning was popularized only in the 1980s, he wrote in his piece for Sky & Telescope.
Centuries ago, "blue moon" had more of a literary meaning of the
absurd, "like saying the Moon is made of green cheese," he wrote.

Hiscock wrote that it was in 1988 when he first heard the term "blue
moon" identified as the second full moon in a month, and was asked about
it after "radio stations and newspapers everywhere carried an item on
this bit of 'old folklore,' as they called it, drawing on an
international wire story."

As it turns out, Hiscock concluded, the term was popularized by the
radio program "Star Date," broadcast in January 1980. By 1985, the
definition was enshrined in "The Kids' World Almanac of Records and
Facts," and shortly after that was a question in Trivial Pursuit in
1986.

A NASA article says
the original source of the "blue moon" definition came after an amateur
astronomer, James Hugh Pruett, was attempting to explain to Sky &
Telescope readers in 1946 what blue moons were, trying to interpret a
complicated definition in the Maine's Farmer Almanac that was "so
convoluted even professional astronomers struggled to understand it." (The simplified, albeit inaccurate, definition stuck, says this Sky & Telescope story.)

The next full moon will occur Sept. 30, which is known as the
harvest moon, and coincides with the Mid-Autumn Festival, known for the
prolific baking of mooncakes in Asian and Asian American communities.

Forecasters expect mostly clear skies in the evening on the coast and
in the Los Angeles Basin, but there could be low clouds and fog later
in the evening. The valleys are expected to be mostly clear.